Squalene, Olive Oil, and Cancer Risk: a Review and Hypothesis

Vol. 6, 1101-1103, December 1997 Cancer Epidemiology,Blornarkers & Prevention 1101

Review

Squalene, Olive Oil, and Cancer Risk: A Review and Hypothesis

Harold L. Newmark’ of unsaturated fatty acids. In Italy, about 80% of edible oil is

Strang Cancer Research Laboratory, The Rockefeller University, New York, olive oil,2 suggesting a protective effect of olive oil intake. In New York 10021, and Laboratory for Cancer Research, College of Pharmacy, another Italian case-control study of diet and pancreatic cancer Rutgers University, Piscataway, New Jersey 08855-0789 (8), increased frequency of edible oil consumption was associated with a trend toward decreased risk. Edible oil in Italy consists of about 80% olive oil, thus suggesting a protective Abstract effect of olive oil in pancreatic cancer.2 Epidemioiogical studies of breast and pancreatic cancer Animal studies on fat and cancer have generally shown in several Mediterranean populations have demonstrated that olive oil either has no effect or a protective effect on the that increased dietary intake of olive oil is associated with prevention of a variety of chemically induced tumors. Olive oil a small decreased risk or no increased risk of cancer, did not increase tumor incidence or growth, in contrast to corn despite a higjier proportion of overall lipid intake. and sunflower oil, in some mammary cancer models (9 -1 1) and Experimental animal model studies of high dietary fat also in colon cancer models (12, 13), although in an earlier and cancer also indicate that olive oil has either no effect report, olive oil exhibited mammary tumor-promoting proper- or a protective effect on the prevention of a variety of ties similar to that ofcorn oil (14), in contrast to the later studies chemically induced tumors. As a working hypothesis, it is (15). proposed that the high squalene content of olive oil, as The protective or, at the least, nonpromoting activity of compared to other human foods, is a major factor in the olive oil is often ascribed to its high content (about 72%) of the cancer risk-reducing effect of olive oil. Experiments in monoenoic unsaturated fatty acid oleic acid (C18:l, o9). But vitro and in animal models suggest a tumor-inhibiting this fatty acid is also found in the fat of beef and poultry in the role for squalene. A mechanism is proposed for the range of 30-45% of the fat and is also found in appreciable tumor-inhibitory activity of squalene based on its known levels in other vegetable oils, such as corn (30%), palm (43%), strong inhibitory activity of 3-hydroxy-fi-methylg1utaryl- peanut (49%), soybean (25%), and sunflower seed (33%; Ref. CoA reductase catalytic activity in vivo, thus reducing 16). The other fats and oils rich in oleic acid are largely farnesyl pyrophosphate availability for prenylation of the associated with increased risk of colon and breast cancer in ras oncogene, which relocates this oncogene to cell humans and generally act as promoters of chemically induced membranes and is required for the signal-transducing tumors in rodents. Thus it seems that the monoenoic unsatur- function of ras. ated fatty acid (oleic acid) content of olive oil cannot account for its protective effect or lack of promotion effect in cancer development. Introduction A recent report of a case-control study of adipose tissue Certain epidemiological studies suggest a cancer-protective ef- stores of individual monounsaturated fatty acids in relation to fect of dietary olive oil relative to other types of nonmarine fat breast cancer in women also suggest that the oleic acid in olive sources. In Greece, women with approximately 40% of energy oil does not account for any protective property of this oil (17). intake from fat, mainly as olive oil, have a breast cancer rate of A consideration was made of other components unique to only about one-third that of United States women, who have olive oil, qualitatively or quantitatively different from other fats until recently also consumed about 40% of energy from fat and oils, as a possible explanation for its protective or nonpro- (1-3). A case-control study in Spain showed a reduced risk for moting effects. Squalene, at up to 0.7%, is uniquely high in breast cancer in women with the highest olive oil consumption olive oil as compared to other common human food fats and (4). In a large case-control study in Greece, a similar lack of oils (18, 19). Other vegetable oils and animal fats are consid- overall association with total fat intake was seen, but breast erably lower, in the range of 0.002-0.03% (18) squalene con- cancer risk was 25% lower in women consuming olive oil more tent. Food-regulatory agencies often rely on the squalene con- than once a day (5). In another case-control study in Spain (6), tent to determine the purity of commercial olive oil. Squalene women in the highest third of monounsaturated fat intake is a hydrocarbon of the triterpene type containing six isoprene (largely from olive oil) had reduced risk of breast cancer units with a pleasant, bland taste. It is a key intermediate in the (relative risk = 0.30; 95% confidence interval, 0.1-0.8). A biosynthetic pathway to steroids in both plants and animals. recent report of a case-control study performed in Italy (7) Thus, it can be considered as almost ubiquitous in most plant indicated a decreased risk of breast cancer with increased intake and animal cells, although at enormously different levels. There are only a few reports of an inhibiting effect of squalene itself in rodent cancer models. Van Duuren and Goldschmidt noted that squalene as well as oleic acid applied Received 6/1 1/97; revised 8/5/97; accepted 8/11/97. The costs of publication of this article were defrayed in part by the payment of topically to mouse skin completely inhibited benzo(a)pyrene- page charges. This article must therefore be hereby marked advertisement in induced skin carcinogenicity (20). o-Limonene only partly in- accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 To whom requests for reprints should be addressed, at Strang Cancer Research Laboratory, The Rockefeller University, 1230 York Avenue, New York, NY 10021. 2 C LaVeccia, personal communication.

hibited skin tumors in the same experiment. In a later study, carcinomas, in which up to 90% are associated with activated Murakoshi et a!. (21) reported that topically applied squalene ras oncogene mutations (28). All these tumors represent po- markedly suppressed the promoting effect of 12-O-tetradeca- tential targets for squalene use as a chemopreventive agent. noylphorbol-13-acetate on mouse skin tumors initiated with Increased dietary squalene intake could theoretically aug- 7,12 dimethylbenz(a)anthracene. In a recent study, we demon- ment cholesterol and bile acid production, resulting in enhanced strated potent inhibition of aberrant hyperproliferation in a atherosclerotic disease. However, a few studies in rabbits and mammary epithelial cell line in vitro (22). In another recent humans indicate that a high intake does not seem to be asso- study using 1% squalene in the diet of rats treated with ciated with a high risk ofatherosclerosis. Kritchevsky et a!. (32) azoxymethane, a 45% suppression of induced aberrant crypt fed 3% squalene in a high-cholesterol diet to rabbits for 14 foci was seen.3 This latter study, to my knowledge, is the first weeks, who failed to develop more atheromas than similar experimental demonstration of the inhibitory action of dietary cholesterol-fed controls. Our recent study also indicates that 1% squalene on carcinogenesis. Other studies on the antitumor squalene in the diet had no effect on serum cholesterol in rats.3 activity of squalene have been reported in Japan (23-25). Strandberg et a!. fed 900 mg of squalene daily to humans for In rats given 1% squalene in the diet for 5 days, serum 7-30 days, demonstrating about 60% absorption with a 17-fold squalene rose about 20-fold. This was accompanied by a strong increase in serum squalene, but produced no consistent in- inhibition (about 80%) of HMG-CoA4 reductase activity in creases in serum cholesterol levels (33). However, these short- hepatic microsomes (26). It is not clear whether this inhibition term feeding studies are insufficient to fully answer questions stems from squalene itself or one or more of its metabolites, of long-term effects of higher-than-normal dietary squalene such as sterols and their oxygenated derivatives (27) produced intake on metabolism of cholesterol and other steroids, as well endogenously. Inhibition of HMG-CoA reductase activity, the as adequacy of biosynthesis of ubiquinones, heme a, and doli- rate-limiting control step in the normal biosynthetic pathway to chols for normal cell function. cholesterol, presumably also reduces the levels of a series of Squalene in humans is supplied by both endogenous bio- intermediates between HMG-CoA reductase and cholesterol synthesis and dietary sources. The Rockefeller University (27) including mevalonate, geranyl pyrophosphate, and FPP. group(18) measured squalene in normal human tissues and IPP is a source for the biosynthesis of ubiquinone, heme a, plasma and found it to be widely distributed, but at great dolichol, and particularly for the prenylation (farnesylation) of variations in level. Sebum was reported to contain 12% squa- certain oncogenes. Many oncogenes such as ras are proteins lene, and adipose tissue contained 0.01-0.04% squalene, with produced in the aqueous environment of the cell cytosol but other tissues having lower levels. Plasma squalene levels rose require relocation to the lipophilic cell plasma membrane for strikingly with increased dietary squalene. Generally, there was activation. This is achieved by attachment of farnesol supplied a direct relationship between plasma levels of squalene and from FPP via a specialized enzyme system (28, 29). Prevention triglycerides, but not with cholesterol. of prenylation prevents the activation of these proteins as The average dietary intake of squalene in the United States signal-transducing agents in the regulation of cell-transforming has been estimated to be only about 30 mg/day (18), with a activity. possible high intake up to 200 mg/day on a 2000-Kcal diet rich Several other dietary isoprenoids are also known to act as in fish and salads dressed with olive oil (18). In Mediterranean inhibitors of the mevalonate pathway and act similarly to re- countries such as Greece, where olive oil intake is about 20 duce cellular availability of FPP, resulting in reduction of tumor kg/year/person (60 g/day), or Italy, where olive oil intake is 12 growth (30). kg/year/person (35 g/day; Ref. 34), average squalene daily There have been several research programs aimed at de- intake is more likely to be in the range of 200-400 mg/thy velopment of (drug) inhibitors of protein farnesylation as po- based on about 0.6-0.7% squalene content (19) of olive oil. tential cancer chemotherapeutic agents (3 1). Some of these This large difference in average dietary intake of squalene agents are targeted to the specific enzymes involved in protein between Mediterranean countries and the United States (7-13- farnesylation, without interfering with the downstream squa- fold) may be related to lower cancer mortality in the Mediter- lene and cholesterol biosynthesis. Dietary squalene presumably inhibits HMG-CoA reductase activity to reduce FPP formation ranean basin countries (1-5). in the cells and also does so without deprivation of any squalene A closely related hypothesis has been proposed by to cholesterol precursors, presumably by enhancing these squa- El-Sohemy et a!. (35), who demonstrated that dietary choles- lene metabolites. terol inhibited chemically induced breast cancer in rats, and The squalene inhibition effect on HMG-CoA reductase they proposed that the administered cholesterol inhibited activity is probably part of normal feedback regulation control HMG-CoA reductase activity, reducing the availability of me- of cholesterol biosynthesis. Increasing dietary intake of squa- valonate, which is required for DNA synthesis and cell prolif- eration. lene (e.g. , from increased intake of olive oil) to augment endogenous levels of squalene thus serves to lower FPP levels and Sharks, which have been claimed to be resistant to cancer therefore reduce oncogene product activation via prenylation, (36), have unusually high tissue levels of squalene. Shark liver and thus it can potentially reduce tumor growth without mark- oil contains 40% or more squalene (18, 19). Dogfish liver oil is edly disturbing a normal biochemical pathway. Reduction of very high, reported to be over 90% squalene (19). It is inter- tumor growth and development could be expected in tumor esting to speculate whether the high squalene content is related types strongly dependent on oncogenes requiring prenylation to the reputed cancer resistance in sharks. for activation, such as ras. Colon, breast, melanoma, and pan- In summary: creas develop such tumor types, particularly pancreatic adeno- (a)Increased dietary intake of olive oil is associated with a small decreased risk or no increased risk of cancer, despite high lipid intake, in epidemiological studies of breast and pancreatic cancer in several Mediterranean populations. 3 C. V. Rao, B. S. Reddy, and H. Newmark, Carcinogenesis, in press. (b) As a working hypothesis, it is proposed that the high 4 The abbreviations used are: HMG-CoA, -hydroxy--methylglutaryl CoA reductase; FFP, farnesyl pyrophosphate. squalene content of olive oil, as compared to other human

foods, is a major factor in the cancer risk-reducing effect of 17. Simonsen, N., Fernandez-Crehuet, J., Martin-Moreno, J., Strain, J., Martin, olive oil. B., Thamm, M., Kardinaal, A., van’t Veer, P., Kok, F., and Kohlmeier, L. Tissue stores ofindividual monounsaturated fats and breast cancer. FASEB J., 11: A578, (c) A few experiments in vitro and in animal models 1997. suggest a tumor-inhibiting role for squalene. 18. Liu, C. K., Ahrens, E. H., Jr., Schreibman, H., and Crouse, R. Measurement (‘0 A mechanism is proposed for the tumor-inhibitory ofsqualene in human tissues and plasma: validation and application. J. Lipid Res., activity of squalene, based on its known strong inhibitory 17: 38-45, 1976. activity of HMG-CoA reductase activity, thus reducing FPP 19. Becker, R. Preparation, composition and nutritional implications of amaranth seed oil. Cereal Foods World, 34: 950-953, 1989. availability for prenylation of some oncogenes (e.g. , ras prenylation as a step in relocation to cell membranes for function 20. Van Duuren, B. L., and Goldschmidt, B. M. Co-carcinogenic and tumor- as signal-transducing agent). promoting agents in tobacco carcinogenesis. J. Natl. Cancer Inst., 56: 1237-1242, 1976. 21. Murakoshi, M., Nishino, H., Tokuda, H., Iwashima, A., Okuzumi, J., Kitano, References H., et a!. Inhibition by squalene of the tumor-promoting activity of 12-0- tetradecanoylphorbol-13-acetate in mouse-skin carcinogenesis. hit. J. Cancer, 52: 1. Gerber, M. Olive oil and cancer. In: A. Giacosa and M. J. Hill (eds.), The 950-952, 1992. Mediterranean Diet and Cancer Prevention. Cosenza, Italy: European Cancer Prevention Organization, 1991. 22. Katdare, M., Singhal, H., Newmark, H., Osborne, M. P., and Telang, N. T. Prevention of mammary preneoplastic transformation by naturally-occurring to- 2. Trichopoulou, A., Toupadaki, N., Tzonou, A., Katsouyanni, K., Manousos, 0., mor inhibitors. Cancer Len., 111: 141-147, 1997. Kada, E., et al. The macronutrient composition of the Greek diet: estimates derived from six case-control studies. Eur. J. Cliii. Nutr., 47: 549-558, 1993. 23. Yamaguchi, T., Nakagawa, M., Hidaka, K., Yoshida, T., Sasaki, T., Akiyama, S., et aL Potentiation by squalene of antitumor effect of 3-[(4-amino- 3. WilIen, W. C. Diet and health: what should we eat? Science (Washington DC), 2-methyl-5-pyrimidinyl)methyl]-l-(2-chloroethy)-nitrosourea in a murine tumor 264: 532-537, 1994. system. Jpn. J. Cancer Res., 76: 1021-1026, 1985. 4. Martin-Moreno, J. M., Willet, W. C., Gorgojo, L, Banegas, J. R., Rodriguez- 24. Ikikawa, T., Umeji, M., Manabe, T., Yanoma, S., Orinoda, K., Mizunuma, Artalejo, F., Fernandez-Rodriguez, J. C., et aL Dietary fat, olive oil intake and H., et a!. Studies on antitumor activity of squalene and its related compounds (in breast cancer risk. bit. J. Cancer, 58: 774-780, 1994. Japanese). J. Pharm. Soc. Jpn., 106: 578-582, 1986. 5. Trichopoulou, A., Katsouyanni, K., Stuver, S., Tzala, L, Gnardellis, C., 25. Ohkima, T., Otagiri, K., Tanaka, S., and Ikekawa, T. Intensification of host’s Rimm, E., et aL Consumption of olive oil and specific food groups in relation to immunity by squalene in sarcoma 180-bearing ICR mice. J. Pharmacobio-dyn., 6: breast cancer risk in Greece. J. Natl. Cancer Inst., 87: 110-116, 1995. 148-151, 1983. 6. Landa, M. C., Frago, N., and Tres, A. Diet and the risk of breast cancer in 26. Strandberg, E., Tilvis, R., and Miettinen, T. A. Variations of hepatic choles- Spain. Eur. J. Cancer Prey., 3: 313-320, 1994. terol precursors during altered flows of endogenous and exogenous squalene in 7. Franceschi, S., Favero, A., Decarli, A., Negri, E., LaVecchia, C., Ferraroni, the rat. Biochem. Biophys. Acts, 1001: 150-156, 1989. M., et aL Intake of macronutrients and risk of breast cancer. Lancet, 347: 1351-1356, 1996. 27. Goldstein, J. L., and Brown, M. S. Regulation of the mevalonate pathway. Nature (Lond.), 343: 425-430, 1990. 8. LaVecchia, C., Negri, E., D’Avanzo, B., Ferraroni, M., Gramenzi, A., Savodelli, R., et ai. Medical history, diet and pancreatic cancer. Oncology (Basel), 28. Kate, K., Cox, A. D., Hisaka, M. M., Graham, S. M., Buss, J. E., and Der, 47: 463-466, 1990. C. J. Isoprenoid addition to Ras protein is the critical modification for its membrane association and transforming activity. Proc. Nail. Acad. Sci. USA, 89: 9. Cohen, L S., Thompson, D. 0.. Mauera, Y., Choi, K., Blank, M. E., and Rose, 6403-6407, 1992. D. P. Dietary fat and mammary cancer. I. Promotion of effect of different fats on N-mtrosomethylurea-induced rat mammary tumorigenesis. J. Nafi. Cancer Inst., 29. Bos, J. L. ras oncogenes in human cancer: a review. Cancer Res., 49: 77: 33-42, 1986. 4682-4689, 1989. 10. Katz, E. B., and Boylan, E. Effect of the quality of the dietary fat in tumor 30. Elson, C. E. Suppression of mevalonate pathway activities by dietary iso- growth and metastasis from a rat mammary adenocarcinoma. Nutr. Cancer, 12: prenoids: protective roles in cancer and cardiovascular disease. J. Nutr., 125: 343-350, 1989. 1666S-l672S, 1995. 11. Lasekan, J. B., Clayton, M. K., Gendron Fitzpatrick, A., and Ney, D. M. 31. Kelloff, 0., Lubet, R. A., Fay, J. R., Steele, V. E., Boone, C. W., Crowd, Dietary olive and safflower oil in promotion of DMBA-induced mammary J. A., and Sigman, C. C. Farnesyl protein transferase inhibitors as potential cancer tumorigenesis in rats. Nutr. Cancer, 13: 153-163, 1990. chemopreventives. Cancer Epidemiol. Biomark. Prey., 6: 267-282, 1997. 12. Reddy, B. S. Dietary fat and colon cancer: animal model studies. Lipids, 27: 32. Kritchevsky, D., Moyer, A. W., Tesar, W. C., Logan, J. B., Brown, R. A., and 807-813, 1992. Richmond, G. Squalene feeding in experimental atherosclerosis. Circ. Res., 11: 13. Rao, C. V., and Reddy, B. S. Modulating effect of amount and types of 340-343, 1954. dietary fat on omithine decarboxylase, tyrosine protein kinase and prostaglandins 33. Strandberg, T. E., Tilvis, R. S., and Miettinen, T. A. Metabolic variables of production during colon carcinogenesis in male F344 rats. Carcinogenesis cholesterol during squalene feeding in humans: comparison with cholestyramine (Land.), 14: 1327-1333, 1993. treatment J. Lipid Res., 31: 1637-1643, 1990. 14. Carroll, K. K., and Khor, H. T. Effects of level and type of dietary fat on 34. Gerber, M. Olive oil and cancer. In: M. J. Hill, A. Giacosa, and C. P. 1. incidence of mammary tumors induced in female Sprague-Dawley rats by 7,12- Caygill (eds.), Epidemiology of Diet and Cancer. New York: Ellis Horwood, dimethylbenz(a)anthracene. Lipids, 6: 415-420, 1971. 1994. 15. Cohen, L. A. In: B. S. Reddy and L. A. Cohen (eds.), Diet, Nutrition and 35. El-Sohemy, A., Bruce, W. R., and Archer, M. C. Inhibition of rat mammary Cancer: A Critical Evaluation, Vol. 1, p. 92. Boca Raton, FL: CRC Press, 1986. tumorigenesis by dietary cholesterol. Carcinogenesis (Lond.), 17: 159-162, 1996. 16. McCance and Widdowson’s. The Composition of Food, 4th ed., pp. 294- 36. Mathews, J. Sharks still intrigue cancer researchers. News report. J. Natl. 299. Elsevier, 1978. Cancer Inst., 84: 1000-1002, 1992.

H L Newmark

Cancer Epidemiol Biomarkers Prev 1997;6:1101-1103.

Updated version Access the most recent version of this article at: http://cebp.aacrjournals.org/content/6/12/1101

E-mail alerts Sign up to receive free email-alerts related to this article or journal.

Reprints and To order reprints of this article or to subscribe to the journal, contact the AACR Publications Subscriptions Department at [email protected].

Permissions To request permission to re-use all or part of this article, use this link http://cebp.aacrjournals.org/content/6/12/1101. Click on "Request Permissions" which will take you to the Copyright Clearance Center's (CCC) Rightslink site.